Method, apparatus and software for analyzing perfusion images

ABSTRACT

The invention relates to a method for analyzing perfusion images, in particular MR perfusion images, of a human or animal organ including the steps of:  
     (a) defining at least one contour of the organ, and  
     (b) establishing at least one perfusion parameter of a region of interest of said organ within a boundary defined by the at least one contour, whereby steps (a) and (b) are repeated in a series of iterative steps wherein for each subsequent iterative step the definition of the at least one contour in step (a) is varied, and the series of iterative steps is terminated after reaching an optimal value for the at least one perfusion parameter in step (b).

[0001] The invention relates to a method for analyzing perfusion images,in particular MR perfusion images, of a human or animal organ includingthe steps of

[0002] (a) defining at least one contour of the organ, and

[0003] (b) establishing at least one perfusion parameter of a region ofinterest of said organ within a boundary defied by the at least onecontour.

[0004] From the article “MR perfusion imaging: correlation with PET andquantitative angiography”, published in Magnetic Resonance Materials inPhysics, Biology and Medicine 11 (2000) 71-72 by J. Sohwitter an G. K.von Schulthess, it is known to apply quantitative tools for obtainingpixelwise slope maps of registered MR perfusion data sets in a methodfor detecting stenosed coronary arteries from the heart. Analysis ofupslope (meaning perfusion rates), rather than other parameters is saidto provide a very sensitive and specific measure of myocardial ischemia.Measuring the upslope provides a semiquantitative measure of absoluteperfusion, even with patients having triple vessel disease.

[0005] The invention aims at improving the method mentioned in theintroductory paragraph.

[0006] To this end, the method of the invention is characterized in thatsteps (a) and (b) are repeated in a series of iterative steps whereinfor each subsequent iterative step the definition of the at least onecontour in step (a) is varied, and the series of iterative steps isterminated after reaching an optimal value for the at least oneperfusion parameter in step (b).

[0007] By this measure, a very accurate determination of an organ'sboundaries is possible allowing for a consequently also very accuratedetermination of the perfusion parameter or parameters that are to beestablished.

[0008] The method of the invention is particularly useful when the organis a heart and the region of interest is the heart's myocardium or asegment thereof.

[0009] It is remarked, however, that the gist of the invention and thescope of protection afforded hereby is not limited to analyzingperfusion images with respect to the heart's myocardium or one or moresegments thereof. The invention may equally well be applied in respectof other organs, for instance the brain. The above and the furtherexplanation below with respect to the analysis of perfusion images ofthe heart, and in particular the heart's myocardium, serves thereforeprimarily to elucidate the method without being restricted to thatsingle application.

[0010] Preferably, in step (a) the inner contour and/or the outercontour of the heart's myocardium is defined. Although one couldrestrict the optimization according to the method of the invention tovariation of the myocardium's inner contour, particularly good resultsare attainable if both the inner contour and the outer contour of thehearts myocardium are varied. This variation may occur simultaneously,but it is preferred to execute the variations subsequently.

[0011] Suitably the perfusion parameter to be established is selectedfrom the group comprising mean upslope, time to peak intensity, time ofcontrast arrival, time to half maximum intensity, accumulated inflow orcombinations thereof. It is found particularly advantageous however thatin step (b) the perfusion rate (or upslope), and/or the time at whichthe maximum perfusion rate occurs is established. These parametersprovide effective indication of the perfusion of the organ beinganalyzed.

[0012] In case the myocardium's inner contour is varied, the method ofthe invention is particularly well applied by clouting the series ofiterative steps and terminating these series after reaching anessentially constant value for the perfusion rate and/or said time atwhich the maximum perfusion rate occurs, as compared to the perfusionrate's value and/or time in a previous iterative step.

[0013] The invention is for embodied in a software program for acomputer of an apparatus that is implemented to execute the method foranalyzing perfusion images as explained above.

[0014] The invention is further embodied in an apparatus that isapparently intended to execute said method.

[0015] The invention shall now be further elucidated with reference tothe following non-limiting exemplary embodiment of the invention withreference to the drawing.

[0016] In the drawing it is shown in:

[0017]FIG. 1: a short axis slice through a heart;

[0018]FIG. 2: intensity time curves of a bolus passage through sectionsof the heart;

[0019]FIG. 3: perfusion parameters varying depending on radialdisplacement of an inner myocardial boundary; and

[0020]FIG. 4: variations of perfusion parameters depending ondisplacements of a myocardial outer boundary.

[0021] In the example, the method for analyzing the perfusion imagesrelates to the myocardium 1 of the left ventricle 2 of a heart (see FIG.1). Reduced blood perfusion of the myocardium 1 of the left ventricle 2is a direct result of cardiovascular diseases. For measuring theperfusion of the myocardium 1, a contrast fluid is applied whilst thepatient's heart is monitored with MR or other imaging techniques. Thatare known per se. The images are obtained by making scans of slices 3through the myocardium 1 during a period of 20 to 40 seconds. In animage sequence, it can be monitored that the contrast fluid first entersthe right ventricle 4, then the left ventricle 2, and finally themyocardium 1.

[0022] The perfusion of these parts of the heart is imaged and theimaging is supported by the contrast agent. The intensity of themeasurement as recorded in time, is shown in FIG. 2. The X-axis of FIG.2 shows time as an independent variable, whilst the Y-axis shows theintensity of the measured perfusion. The Figure clearly shows that thecontrast agent first enters the right ventricle. This is shown by graphA. Subsequently, the contrast agent arrives at the left ventricle asshown by graph B. Finally the contrast agent arrives at the myocardiumas shown by graph C. The intensity time curve C pertaining to themyocardium is used for analyzing the perfusion of the myocardium.

[0023] Since the signal in the myocardium 1 is very noisy, generally themyocardium 1 is divided into segments and the measurements are averagedover these segments. These segments preferably coincide with areas ofthe myocardium 1 which are supplied with blood from a certain coronaryartery. In this way, if a reduced perfusion is observed in a myocardialsegment, it can be traced back to the supplying artery.

[0024] Since the signal strengths in the left 2 and right 4 ventriclesare much higher than in the myocardium 1, extreme care should be takennot to include any part of the left and right ventricles into themyocardial segments. This is not as easy as it seems, since theboundaries of the left and ventricle blood volumes can be very irregulardue to the presence of papilar muscles and trabeculae.

[0025] Also the boundaries of the myocardium 1 are often not clearlyvisible in a single image of the sequence or even on a maximum intensityprojection through time. Furthermore, the images generally have a ratherlow resolution (typically 128*128 pixels) and only a few pixels areavailable for averaging within a segment.

[0026] The parameters that are of interest in analyzing the perfusionare on the one hand the perfusion rate or upslope as indicated in FIG. 2with line D in relation to graph C, and on the other hand the time t_(u)at which the maximum perfusion rate occurs. FIG. 2 relates to a specificsituation with a fixed inner boundary and outer boundary of themyocardium 1.

[0027]FIGS. 3 and 4 show the results with a varying diameter of innercontour and outer contour of myocardium 1 respectively, and theresulting value of the maximum upslope or maximum perfusion rate, andthe time at which this maximum perfusion rate occurs (see FIGS. 3 and 4left and right respectively).

[0028] The X-axis of FIGS. 3 and 4 shows as the independent variable avariation of the concerning contour of the myocardium (FIG. 3: innercontour; FIG. 4: outer contour).

[0029] From FIG. 3 is follows that when the myocardium's inner contouris varied, eventually the above-mentioned perfusion parameters reach anessentially constant value, whilst further variation of the innercontour, i.e. a diminishing diameter beyond the point at which theconstant value is reached, does no longer effect either the maximumperfusion rate or the time at which such maximum perfusion rate occurs.The point beyond which fisher variation of the inner contour does nolonger effect the perfusion parameters is then established to representthe measured inner contour that most accurately corresponds to the trueboundary of the myocardium.

[0030] Likewise, when varying the outer contour of tae myocardium, anoptimal value of both the perfusion rate and the time at which themaximum perfusion rate occurs, can be established (see FIG. 4). Theouter contour value corresponding to these optimal values is then takento accurately represent the actual outer boundary of the myocardium.

[0031] When one or more optimal values of the said perfusion parametersare established, further iteration by repeatedly defining a contour ofthe organ and subsequently establishing at least one perfusion parameterof the analyzed region of interest of the organ can be terminated. Inthis way, an accurate method for detecting the myocardial contours isdefined that ensures that no part of the left ventricle 2 or rightventricle 4 is included in the perfusion images of the myocardium 1.

1. Method for analyzing perfusion images, in particular MR perfusionimages, of a human or animal organ including the steps of (a) definingat least one contour of the organ, and (b) establishing at least oneperfusion parameter of a region of interest of said organ within aboundary defined by the at least one contour, characterized in thatsteps (a) and (b) are repeated in a series of iterative steps whereinfor each subsequent iterative step the definition of the at least onecontour in step (a) is varied, and the series of iterative steps isterminated after reaching an opts value for the at least one perfusionparameter in step (b).
 2. Method according to claim 1, characterized inthat the organ is a heart and the region of interest is the heart'smyocardium or a segment thereof.
 3. Method according to claim 2,characterized in that in step (a) the inner contour and/or the outercontour of the heart's myocardium is defined.
 4. Method according to anyone of claims 1-3, characterized in that in step (b) the perfusion rateor upslope and/or the time at which the maximum perfusion rate occurs isestablished.
 5. Method according to claim 4, characterized in that instep (b) the myocardium's inner contour is varied and that the series ofiterative steps is terminated after reaching an essentially constantvalue for the perfusion rate and/or said time at which the maximumperfusion rate occurs, as compared to the perfusion rate's value and/ortime in a previous iterative step.
 6. Software program for a computer ofan apparatus implemented to execute a method for analyzing perfusionimages, in particular MR perfusion images, of a human or animal organincluding the steps of (a) defining at least one contour of the organ,and (b) establishing at least one perfusion parameter of a region ofinterest of said organ within a boundary defined by the at least onecontour, characterized in that steps (a) and (b) are repeated in aseries of iterative steps wherein for each subsequent iterative step thedefinition of the at least one contour in step (a) is varied, and theseries of iterative steps is terminated after reaching an optimal valuefor the at least one perfusion parameter in step (b).
 7. Soft programaccording to claim 6, characterized in that in step (a) the innercontour and/or the outer contour of the heart's myocardium is defined.8. Software program according to claim 6 or 7, characterized in that instep (b) the perfusion rate or upslope and/or the time at which themaximum perfusion rate occurs is established.
 9. Software programaccording to claim 8, characterized in that in step (b) the myocardium'sinner contour is varied and that the series of iterative steps isterminated after reaching an essentially constant value for theperfusion rate and/or said time at which the maximum perfusion rateoccurs, as compared to the perfusion rate's value and/or time in aprevious iterative step.
 10. Apparatus for analyzing perfusion images,in particular MR perfusion images, of a human or animal organ, arrangedto execute the steps of (a) defining at least one contour of the organ,and (b) establishing at least one perfusion parameter of a region ofinterest of said organ within a boundary defined by the at least onecontour, characterized in that during operation steps (a) and (b) arerepeated in a series of iterative steps wherein for each subsequentiterative step the definition of the at least one contour in step (a) isvaried, and the series of iterative steps is terminated after reachingan optimal value for the at least one perfusion parameter in step (b).11. Apparatus according to claim 10, characterized in that in step (a)the inner contour and/or the outer contour of the heart's myocardium isdefined.
 12. Apparatus according to claim 10 or 11, characterized inthat in step (b) the perfusion rate or upslope and/or the time at whichthe maximum perfusion rate occurs is established.
 13. Apparatusaccording to claim 12, characterized in that in step b) the myocardium'sinner contour is varied and that the series of iterative steps isterminated after reaching an essentially constant value for theperfusion rate and/or said time at which the maximum perfusion rateoccurs, as compared to the perfusion rate's value and/or time in aprevious iterative step.